The demonstration that stem cells exist in the adult brain and spinal cord (Chiasson et al. J. Neurosci. 1999 19:4462-71; Doetsch et al. Cell 1999 97:703-16; Gage et al. J. Neurobiol. 1998 36:249-66; Johansson et al. Exp. Cell Res. 1999 253: 733-6; Kukekov et al. Exp. Neurol. 1999 156:333-44; Pagano et al. Stem Cells 2000 18:295-300; Palmer et al. J. Neurosci. 1999 19:8487-97; Weiss et al. J. Neurosci. 1996 16:7599-609), that neurogenesis and gliogenesis are ongoing processes (Eriksson et al. Nat. Med. 1998 4:1313-7; Horner et al. J. Neurosci. 2000 20:2218-28; Johansson et al. Cell 1999 96:25-34; Kirschenbaum et al. Cereb. Cortex 1994 4:576-89) and that stem cell populations can be modulated by extrinsic signals (Ahmed et al. J. Neurosci. 1995 15:5765-78; Forsberg-Nilsson et al. J. Neurosci. Res. 1998 53:521-30; Johe et al. Genes Dev. 1996 10:3219-40; Kalyani et al. Dev. Biol. 1997 186:202-23; Palmer et al. J. Neurosci. 1999 19:8487-97; Tsai, R. Y. and McKay, R. D. J. Neurosci 2000 20:3725-35; Vescovi et al. Exp. Neurol. 1999 156:71-83; Weiss et al. J. Neurosci. 1996 16:7599-609), has lead to a plethora of publications characterizing multipotent neural stem cells (NSCs) (Johansson et al. Exp. Cell Res. 1999 253:733-6; Kalyani et al. Dec. Biol. 1997 186:202-23; Morrison et al. Cell 1999 96:737-49; Reynolds, B. A. and Weiss, S. Dev. Biol. 1996 175:1-13; Stemple, D. L. and Anderson, D. J. Cell 1992 71:973-85; Vescovi et al. Exp. Neurol. 1999 156:71-83; Weiss et al. J. Neurosci. 1996 16:7599-609). What has become clear is that several classes of multipotent cells exist, all of which are nestin immunoreactive and capable of differentiating into astrocytes, neurons and oligodendrocytes. Different populations of cells can be distinguished by differences in culture conditions, self-renewal capability, as well as in their ability to integrate and to differentiate following transplantation (Gage, F. H. Science 2000 287:1433-8; Rao, M. S. Anat. Rec. 1999 257:137-48).
Rodents NSCs isolated from different regions of the rostrocaudal axis and at different developmental stages exhibit differences in differentiation potential, growth factor dependence and gene expression. For example, stem cells isolated at an early stage of embryogenesis from the developing spinal cord appear to require fibroblast growth factor (FGF) for survival, while stem cells isolated from more rostral portions at later developmental stages seem equally responsive to FGF and/or to epidermal growth factor (EGF) (Reynolds, B. A. and Weiss, S. Dev. Biol. 1996 175:1-13). Cells isolated from the ventricular zone express GFAP as a marker in the adult (Doetsch et al. Cell 1999 97:703-16) while multipotent cortical stem cells express polysialated NCAM (Marmur et al. Dev. Biol. 1998 204:577-91). Responses to neurotransmitters also appear different. Ventricular zone stem cells proliferate in response to glutamate while subventricular zone stem cell turnover is reduced (Haydar et al. J. Neurosci 2000 20:5764-74). FGF-dependant stem cells can generate EGF-dependent cells in vitro, suggesting that these cells may represent different developmental stages. The lineage relationship between these various cells remains to be determined.
Less is known about human neural stem cells (hNSCs) isolated from fetal and adult tissue (Brannen, C. L. and Sugaya, K. Neuroreport 2000 11:1123-8; Carpenter et al. Exp. Neurol. 1999 158:265-78; Flax et al. Nat. Biotechnol. 1998 16:1033-9; Johansson et al. Exp. Cell Res. 1999 253:733-6; Kukekov et al. Exp. Neurol. 1999 156:333-44; Pagano et al. Stem Cells 2000 18:295-300; Vescovi et al. J. Neurotrauma 1999 16:689-93; Vescovi et al. Exp. Neurol. 1999 156:71-83; VIIIa et al. Exp. Neurol. 2000 161:67-84). These cells give rise to glia and neurons, can be grown under different culture conditions and show different growth factor requirements. The lineage relationship among the various identified hNSCs and their relationship to previously described rodent stem cell populations remains to be determined. Indeed, comparative studies of rodent and human-derived stem cells have been hampered by the limited availability of cross-reactive reagents. For example, the monoclonal antibody nestin does not react with human cells and AC133, a recently identified stem cell marker, does not cross react with rat-derived NSC's (Uchida et al. Keystone Symposium 2000).
Recently, hNSCs have become available through commercial sources such as Clonetics (San Diego, Calif.) and Clonexpress (Gaithersberg, Md.). First passage cells and growth conditions are available that generate neurons, astrocytes and possibly oligodendrocytes. Cells from Clonexpress have a limited differentiation capacity and generate cells that co-express neuroglial markers (Piper et al. J. Neurophysiol. 2000 84:534-48).
Methods have now been developed for isolating and propagating a tripotential human precursor cell which, upon characterization, has been shown to share many features with a rodent-derived neuroepithelial precursor (NEP) (Kalyani et al. Dev. Biol. 1997 186:202-231 Kalyani et al. J. Neurobiol. 1999 38:207-24). Like rodent-derived NEPs, hNEPs can be grown as adherent cultures in FGF/chick embryo extract (CEE) and do not require leukemia inhibitory factor (LIF) or EGF for proliferation and survival. These cells can be induced to differentiate into astrocytes, neurons and oligodendrocytes in culture. A subset of the hNEPs express AC133 and a small percentage are also GFAP positive. Further, hNEPs transplanted into the intact adult rat brain survive, can be identified with human-specific antibodies, proliferate, differentiate into neurons and glia and migrate extensively in a context dependent manner.